There are two basic techniques for implementing Analog-to-Digital Converters (ADCs), the open-loop technique and the feedback technique. An open-loop converter generates a digital code directly upon application of an input voltage, and is generally asynchronous in operation. A feedback converter generates a sequence of digital codes from an input signal and reconverts these digital codes to an analog signal.
Sigma-delta ADCs use the feedback technique. The sigma-delta technique is attractive because it achieves high resolution by precise timing instead of precisely matched on-chip components, such as resistors and capacitors used in open-loop converters. Thus, the sigma-delta technique is the technique of choice for many integrated circuit applications.
A basic sigma-delta ADC receives an analog input signal and subtracts a feedback signal from the analog input signal to provide an error signal. The error signal is processed through a lowpass filter and then quantized to form a digital output signal. A feedback Digital-to-Analog Converter (DAC) provides the feedback signal after converting the digital output signal to analog form. Aside from the feedback DAC, the basic sigma-delta ADC may be implemented with conventional analog components such as operational amplifiers, comparators, and switched-capacitor filters. The basic sigma-delta ADC usually provides high resolution because integrated circuit clocking speeds allow the analog input signal to be highly oversampled. The basic sigma-delta ADC also has a high Signal-to-Noise Ratio (SNR) because the lowpass filter shapes quantization noise out-of-band, which can then be sufficiently attenuated by conventional filtering techniques.
While the basic sigma-delta ADC is easy to implement in conventional integrated circuit processes and generally has high performance, it is not ideal for some applications. For example, a receiver having a basic sigma-delta ADC is not ideal for receiving multiple communication protocols such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Global System for Mobile communication (GSM), and Advanced Mobile Phone Service (AMPS). These protocols each have different information-bandwidths, e.g., the information-bandwidth of a CDMA signal is approximately 1 megahertz (MHz), whereas the information-bandwidth of a GSM signal is approximately 200 kilohertz (kHz). A problem with a receiver having a basic sigma-delta ADC for receiving multiple communication protocols is that the dynamic range of the sigma-delta ADC varies with the information-bandwidth of the received signal. The dynamic range of the sigma-delta ADC decreases as the information-bandwidth of the received signal increases.
Accordingly, it would be advantageous to have a sigma-delta ADC and method for receiving multiple protocols. It would be of further advantage for the sigma-delta ADC to have an adjustable information-bandwidth while maintaining the dynamic range of the sigma-delta ADC.